There has conventionally been a technique of distributing a group key to each node in a network forming a directed acyclic graph topology, which is composed of plural nodes including one node serving as a root node and having a parent-child relationship among nodes of adjacent hierarchies. The group key is used for authentification in order to confirm whether a communication to each node connected to the network is possible or not. In this technique, the group key is distributed to each node from the root node (edge router) as being encrypted. In this case, a group key distribution message containing the encrypted group key is transferred to low-order nodes from high-order node. An encryption key used for the encryption upon the distribution of the group key is created when a network access authentification, executed between each node and the root node when each node joins the group, is successful. The group key is updated every time the node connected to the network is changed, and a group key update message containing the group key, which is encrypted with respect to each node that can be communicated in the network, is distributed as in the group key distribution message.